Preheat oven to 350˚F and line muffin tins with liners or lightly grease.
In a large bowl, beat together the butter and sugar until light and fluffy. Beat in the egg, then the egg yolk. Mix in half the all-purpose flour and baking powder until just incorporated. Mix in the sour cream and water until just combined, then the remaining flour. Stir in the vanilla extract, then divide the batter among prepared muffin tins.
If desired, slowly add cocoa powder to remaining mixture to create a chocolate batter.
Bake the cupcakes in preheated oven until set and toothpick inserted in center comes out clean, 20-23 minutes. Cool in the tins for 5 minutes before removing the cupcakes to a wire rack to cool to room temperature.